Party today, learning curve

I was in a pretty good party today. Except me, duh! For some reason, twice, I managed to target a leech instead of the party's target and doomed us all. Man I hate it when that happens. I try so hard to be sure I am targeting the parties target and boom something jumps in front of me and I hit it instead. grrrrrrrrrr. Not matter how hard I try. Learning curve I suppose.